Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, such as mobile broadband usage and possibilities for competitive offerings to customers, a need for increased capacity for conveying the data is emerging. Thus, techniques which allow mobile operators to manage their spectrum resources efficiently are of high importance.
Therefore, in order to support uplink data rates, mobile operators may provide their base stations with additional receiving antennas. Base stations equipped with multiple antennas may improve the downlink performance by introducing support for four antenna branch MIMO transmission. In addition to doubling the peak data rate when compared to two antenna branch MIMO transmission, the possibility of transmitting from four antennas will also increase the coverage for rank-1 and rank-2 transmissions. Therefore, 4-branch MIMO transmission schemes are well applicable for increasing the cell and cell-edge user data rates.
Prior art which is related to this technical field can e.g. be found in technical specifications according to 3GPP Release 11, in particular, the Draft Report of 3GPP TSG RAN WG1 #69 v0.2.0.
According to the above mentioned specification, it has been agreed that in case of re-transmissions, relative to the initial transmission of a codeword:                Number of layers (and transport blocks TB) per codeword CW shall be maintained        Order of codewords CW shall be maintained        Special mappings for re-transmissions are not considered further        
This allows for a straight forward extension of the standard to 4Tx MIMO. However, in some particular cases the agreements above can cause suboptimal behavior.
That is, currently, 4-branch MIMO is standardized in 3GPP within Rel 11, wherein in previous 3GPP meetings, the usage of so called codewords CW was decided. A codeword is the combination of up to two transport blocks. The number of transport blocks in one codeword depends on the rank. Only one acknowledgement/negative acknowledgement Ack/Nack is signaled per codeword, i.e. if one transport block in the codeword is not successfully transmitted, the whole codeword has to be retransmitted. That also implies that a CW with two transport blocks can only be retransmitted in the same format (CW with two transport blocks), and codewords with one transport block can only be retransmitted in a 1-transport block per CW format.
The problem is the rank reduction in case of re-transmissions. From the description above, it can be concluded that if a CW with two transport blocks fails, it can be retransmitted with rank 3 or 4 but not with rank 1 or 2. A codeword with two transport blocks can't be simply mapped to two codewords with one transport block each.
Another problem is that each hybrid automatic repeat request HARQ has an identification ID. Retransmitting for example CW 1 from a rank 4 transmission as CW 2 in rank 3 would require the definition of a special mapping of the HARQ ID.
This is technically feasible but would require undesired exceptions in the standard. The conclusion is that rank reduction for re-transmission is complicated and standard will not introduce explicit mechanisms for supporting it.
The current opinion in 3GPP for those re-transmissions is that re-transmissions should keep the rank. If this rank is too optimistic and the re-transmissions are not successful, the base station NB would simply terminate the HARQ process and start the transmission of the affected transport blocks from scratch.
However, for some cases, this configuration may suffer from problems. That is, as an example, assuming the user equipment UE gets a rank 4 transmission (two CWs, each carrying two transport blocks), one CW succeed and is acknowledged, and the other one CW fails and gets a negative acknowledgement Nack requesting for a re-transmission. As the rank needs to be maintained for re-transmissions, the CW to be retransmitted needs to be accompanied with another CW delivering new data. By the time the re-transmission is to take place the channel may have gotten worse (UE is moving away from NB, or rank 4 was scheduled during an exceptional good transmission time interval TTI) and it is quite likely that the CW with new data also fails (only one of the two contained transport blocks has to fail). The NB could now terminate the 1 HARQ process after the maximal number of re-transmissions is reached for this CW. However, the second CW would still require re-transmissions. And two new transport blocks would be scheduled for the first CW. Since the channel conditions are not adequate for rank 4, the new CW 1 would also fail. In this manner, the UE can be stuck in rank 4 re-transmissions.
Another problematic case is when the NB transmit buffer is empty, it has CW to be retransmitted with high rank, but no new data to transmit, and hence it is not able to accompany the CW to be retransmitted with another CW carrying new data, preventing the re-transmission to take place at the same rank it was initially transmitted.